Almost every material has a substantial, positive coefficient of thermal expansion, i.e. it expands significantly when heated. Since World War II, the importance of low expansion ceramics has been realized and extensive research has been conducted to develop materials that exhibit low thermal expansion. Some of the well know materials include vitreous silica, lithium alumino-silicates (LiAlSiO.sub.4, LiAlSi.sub.2 O.sub.6), and cordierite (Mg.sub.2 Al.sub.4 Si.sub.5 O.sub.18). Vitreous silica is used in a variety of low temperature applications such as various optical systems. Lithium alumino-silicates (LAS) are used as a major constituent of cooking ware; and cordierite as the main component of catalytic substrate bodies. However most of these materials have temperature limitations, i.e. they cannot be used at very high temperatures.
Components of catalytic converters used in automobiles as well as industrial emission control systems are subjected to very large temperature variations. These components are therefore made out of low thermal expansion and highly thermal shock resistant ceramic materials. Current state of the art materials include aluminum titanate, and cordierite. However, aluminum titanate possesses a structural instability (decomposition into rutile and corundum) below 1300.degree. Centigrade (C), destroying the pseudobrookite structure and resulting in a loss of low thermal expansion behavior. Cordierite, due to its relatively low melting temperature, cannot be used above 1200.degree. C.
Some of the components of earth orbiting satellites are also subject to temperature fluctuations. If these components are made out of materials having a large thermal expansion, then sudden temperature changes can cause the component to distort or fracture under thermal stresses. This problem is of a great concern in optical systems used in space that cannot function satisfactorily under frequently changing dimensions of the component.
Ceramics of the type Na.sub.1+X Zr.sub.2 P.sub.3-X Si.sub.X O.sub.12 (also known as NASICON) and NaZr.sub.2 P.sub.3 O.sub.12 (or NZP) have been studied for their ionic conductivity. Sljukic, et al. were the first to synthesize NZP-type materials ("Preparation and Crystallographic Data of Phosphates with Common Formula M.sup.I M.sup.IV (PO.sub.4).sub.3 ; M.sup.I =Li, Na, K, Rb, Cs; M.sup.IV =Zr, Hf; Croatia Chemica Acta, 39, pp. 145-148, 1967) by heating a mixture of alkali metal phosphates and tetravalent metal oxides. The crystal structure of NZP type materials consists of a three-dimensional hexagonal skeleton network of PO.sub.4 tetrahedra connected with ZrO.sub.6 octahedra by corner linking. Each PO.sub.4 tetrahedron is connected to four ZrO.sub.6 octahedra and each ZrO.sub.6 octahedron is connected with six PO.sub.4 tetrahedra. The basic unit of the network consists of two octahedra and three tetrahedra corresponding to (Zr.sub.2 P.sub.3 O.sub.12). These units in turn are so connected as to form ribbons along the c-axis, that are joined together (perpendicular to the c-axis) by PO.sub.4 tetrahedra to develop a three dimensional rigid network. The articulation of these ribbons and chains is believed to create structural holes or interstitial vacant sites in the structure which are normally occupied by sodium and/or other substituting ions. There are four such interstitial sites per formula unit of which some are empty depending upon the particular substitution.
The most important and extraordinary feature of the NZP structure is its exceptional flexibility towards ionic substitution at various sites. This feature is extremely important for manipulating the thermal expansion behavior of NZP type materials. J. P. Boilot and J. P. Salantie, as reported in "Phase Transformation in Na.sub.1+X Zr.sub.2 P.sub.3-X Si.sub.X O.sub.12 Compounds", Materials Research Bulletin Vol. 14, pp.1469-1477, 1979, found that the thermal expansion of various compositions in Na.sub.1+X Zr.sub.2 P.sub.3-X Si.sub.X O.sub.12 varied from strongly positive to near zero to even negative values. Based on these results J. Almo and R. Roy further investigated the NZP type materials and showed that NZP type materials indeed show low thermal expansion and have extraordinary flexibility towards ionic substitutions ("Ultralow Expansion Ceramics in the System Na.sub.2 O--ZrO.sub.2 --P.sub.2 O.sub.5 --SiO.sub.2, "Journal of American Ceramic Society, Vol 67 No.5 pp. C-78-C-79 1984; "Crystal Chemistry of the NaZr.sub.2 (PO.sub.4).sub.3, NZP or CTP Structure Family," Journal of Materials Science, vol. 21 pp.444-450 1986).
Later investigations on NZP type materials were performed by D. K. Agrawal and V. S. Stubican ("Synthesis and Sintering of Ca.sub.0.5 Zr.sub.2 P.sub.3 O.sub.12 --A Low Thermal Expansion Material," Materials Research Bulletin, vol. 20 No 2 pp. 99-106, 1985) reporting the sintering characteristics of CaZr.sub.4 (PO.sub.4).sub.6. T. Oota and I. Yamai in a publication entitled "Thermal Expansion Behavior of NaZr.sub.2 (PO.sub.4).sub.3 --Type Compounds," Journal of the American Ceramic Society, vol 69 No.1 pp. 1-6 (1986) suggested that if larger ions such as K.sup.+ or Sr.sup.2+ are substituted at the sodium site, then the c-axis will be stretched. The PO.sub.4 tetrahedra that cross link the chains along the c-axis are strained upon stretching of the c-axis. Hence, during heating, further expansion along the c-axis is suppressed, and the expansion along the a-axis is enhanced. They also concluded that in NZP type materials, the skeletal framework thermal expansion is almost zero, and that the total expansion was mainly determined by the ionic substitution at different crystallographic sites.
Recently S. Y. Limaye, D. K. Agrawal and H. A. McKinstry ("Synthesis and Thermal Expansion of MZr.sub.4 P.sub.6 O.sub.24 (M=Mg, Ca, Sr, Ba)," Journal of the American Ceramic Society Vol. 70 No. 10 pp. C-232-C-236 1987) conducted a systematic survey of NZP type materials with alkaline earth ions substituted at the sodium site. They found that MgZr.sub.4 P.sub.6 O.sub.24 did not result in a structure similar to NZP, that CaZr.sub.4 P.sub.6 O.sub.24 had positive thermal expansion along the c-axis and negative thermal expansion along a-axis, and that SrZr.sub.4 P.sub.6 O.sub.24 and BaZr.sub.4 P.sub.6 O.sub.24 showed exactly opposite thermal expansion anisotropy This work was further extended by making a solid solution of CaZr.sub.4 P.sub.6 O.sub.24 and SrZr.sub.4 P.sub.6 O.sub.24 which showed reduced thermal expansion anisotropy and also reduced bulk thermal expansion. However, most of the earlier research has been concentrated on making ionic substitutions at sodium and zirconium sites. Earlier research did not address the following issues: the thermal expansion anisotropy (except for the solid solution of CaZr.sub.4 P.sub.6 O.sub.24 and SrZr.sub.4 P.sub.6 O.sub.24),effect of the thermal expansion anisotropy on various properties (including Young's modulus, strength etc.) characterization of the thermal expansion up to higher (e.g. 1200.degree. C.) temperatures, evaluation of cyclic thermal expansion, strength and thermal shock resistance, efforts to improve the strength and thermal shock resistance and non-linear thermal expansion, i.e. large variation in thermal expansion with respect to temperature. Furthermore, most of these materials have relatively higher thermal expansion than fused quartz up to 500.degree. C.